1. Field of the Invention
This invention relates to a projector such as a video projector, a liquid crystal projector or the like, and particularly to a projector for projecting images of image forming means which correspond to the colors of, for example, B (blue), G (green) and R (red), onto a screen in an overlapping fashion to form a composite multicolor image.
2. Description of the Prior Art
FIG. 71 shows the optical system of a conventional projector. The projector in FIG. 71 has image forming means 1, 2, 3 such as a liquid crystal display (hereinafter referred to as the "LCD") or as a CRT. The image forming means form pictures, and projecting lenses 4, 5 and 6 having optical axes Ax1, Ax2, Ax3 are perpendicular to each image forming means. The image forming means 1 and the lens 4 comprise a projecting optical system A. The image forming means 2 and the lens 5, and the image forming means 3 and the lens 6 comprise projecting optical systems B, C. In FIG. 71, principal rays emitting from three points of each image forming means are shown.
The optical axis Ax2 of the central projecting lens 5 is perpendicular to a screen 7 onto which images are projected, while the optical axes Ax1, Ax3 of the projecting lenses 4, 6 intersect with the optical axis Ax2 of the projecting lens 5 on the screen and are not perpendicular to the screen 7.
It is defined here that the optical axis Ax1 is the x-axis, the crossline between a plane including three optical axes and the screen is the y-axis, and the direction perpendicular to the y-axis on the screen is the z-axis.
FIG. 72 shows an optical path of the projecting optical system C of FIG. 71. The luminous flux as shown in this Figure is converged most in the projecting lens 6.
However, the above-mentioned conventional projector has problems since images formed by the projecting optical systems A, C are angled with respect to the screen. As a result, distortion is generated and a focus error of an image is generated in the peripheral portion in the y-axis direction.
Next, the degree of focus error in the above-mentioned construction will be described concretely by applying concrete numerical figures.
The image forming means is an LCD of three inches size. The display area is about 46 mm.times.61 mm. The LCD is provided on the periphery of the display area with a lead frame or a substrate for mounting a drive IC. In this example, the substrate measures 160 mm in the width direction. When, therefore, the LCDs are arranged side by side as shown in FIG. 71, the distance between the centers of the adjacent LCDs is a minimum of 160 mm. Also, it is arranged such that the focal lengths of the projecting lenses 4, 5, 6 are 75 mm, the magnification is -15.5 times, the distances from the image forming means to the corresponding projecting lenses are 79.8 mm, the distances from the projecting lenses to the screen are 1237.5 mm, and the distances from the central projecting lens 5 to the peripheral projecting lenses 4,6 are 160 mm, respectively. According to this arrangement, the angle formed between the optical axis Ax2 of the central projecting lens 5 and the optical axes Ax1, Ax3 of the projecting lenses 4, 6 is 7.4.degree..
FIG. 73 shows a distortion and spot diagram of the image when lattice image forming means are projected to the screen by the projecting optical systems B, C. Since the spot diagram appears symmetrically with reference to the y-axis, only one side is shown in the Figure.
A lattice indicated by broken lines in the Figure is an image projected by the system C, while the lattice indicated by a solid line is an image projected by the system B. Since the projecting optical system C has such distortion, a point expressed by the coordinate (y , z)=(30.5,22.9) on the LCD 12 is imaged at a point of (y , z)=(-454.0,-337.7) on the screen, while a point expressed by the coordinate (y , z)=(-30.5,22.9) on the LCD 12 is imaged at a point (y , z)=(501.1,-372.7) on the screen. If there were no distortion of the image, the point on the LCDs should be imaged at points (y , z)=(.+-.472.4,-354.3) on the screen.
The dots in the Figure show dispersion of luminous flux on each point. If the image plane coincides with the screen, that is, if there is no focus error at any points, luminous flux is focused into one point. The size of the dot corresponds to the focus error of the image at the relevant points. FIG. 73 shows the dispersion of the luminous flux enlarged by 20 times.
A projection image projected by the other peripheral projecting optical system A generates a focus error and a line distortion symmetric with the image formed by the projecting optical system C reference to the z-axis.
In order to reduce the focus error of an image, there has also been proposed a projector of the type shown in FIG. 74.
The luminous flux having the components R, G and B and coming from the image forming means 1, 2 and 3 are overlapped by a dichroic prism 8 and projected to the screen 7 by the projecting lens 9. In order to overlap the luminous flux, a dichroic mirror is also used besides the dichroic prism 8.
According to this method, since the luminous flux from each image forming means is projected onto the screen 7 by a single projecting lens, no focus error and distortion are generated.
However, in the construction shown in FIG. 74, the parallel luminous flux is made incident to the dichroic prism and the prism is required to be the same size as the image forming means. If, therefore, the size of the image forming means is made large in order to improve the resolution of the image, the prism and the projecting lens must also be large and high cost results.
The same problem is present when a dichroic mirror is used.